This invention relates to methods for making field effect transistors and more particularly to a method for making such transistors with one or more narrow channels.
Recently, there has been much development of insulated gate field effect transistors (IGFET) which use metal insulator semiconductor (MIS) technology. The benefits of this technology include simplicity of structure and the potential for very high device density.
As such devices become increasingly smaller, it is increasingly more desirable to operate at low currents. Unfortunately, the drain to source current is proportional to the width-to-length (W/L) ratio of the device channel. One illustration of this dependency is the MOS transistor current equation, e.g., for saturation: EQU I.sub.DS =-k'(W/L)(V.sub.GS -V.sub.T).sup.2 ( 1)
where
I.sub.DS =drain to source current PA0 W=channel width PA0 L=channel length PA0 V.sub.GS =gate to source voltage PA0 V.sub.T =threshold voltage PA0 k'=.mu.p.epsilon.ox/2t.sub.ox PA0 .mu..sub.p =avg. surface mobility of channel holes PA0 .epsilon..sub.ox =oxide permitivity PA0 t.sub.ox =thickness of the oxide over the channel.
Obviously, the drain-to-source current could be decreased by increasing the channel length dimension relative to the width, but the disadvantages of the resulting decrease in density, decrease in operational speed, etc. would probably outweigh the benefits from the decreased current.
Another approach to decreasing the W/L ratio is to decrease the width itself. However, the lower limits of channel width are conventionally limited by the photolithographic techniques used to fabricate such devices. Typically, photolithographic resolution is in the range of several (greater than 3) micrometers.
The microelectronics industry has successfully employed several techniques to avoid the limitations of photolithography and to control channel dimensions. One approach involves using a double diffusion technique, typically in which channel-forming impurities of one conductivity type are diffused into a relatively large region of the substrate, then source- or drain-forming impurities of the second conductivity type are diffused into the channel region, but to a smaller area, to precisely define a relatively short channel between the lateral edges of the diffused regions. U.S. Pat. No. 3,845,495 issued Oct. 29, 1974 to Cauge et al. uses the same oxide mask aperture for both diffusions, and uses the relative lateral diffusion of the source and channel regions to control the positioning of their lateral edges. U.S. Pat. No. 3,883,372 issued May 13, 1975 to Lin uses a glass mask having concentric apertures and apparently controls the lateral edges of the diffused regions by the extent of lateral diffusion, in the manner of Cauge et al. U.S. Pat. No. 3,863,330 issued Feb. 4, 1975 to Kraybill et al. uses the same nitride mask aperture for both diffusions and again controls the channel length by relative lateral diffusion. U.S. Pat. No. 4,038,107 issued July 26, 1977 to Marr et al. forms the source through an oxide mask aperture, forms the channel through a smaller polysilicon mask aperture, oxidizes the polysilicon mask, and then forms the drain through the still smaller, oxidized polysilicon mask aperture.
In another approach, U.S. Pat. No. 4,037,307 issued July 26, 1977 to Smith uses a mask of abutting silicon dioxide and metal oxide and forms an aperture therein by etching the silicon dioxide away from the silicon dioxide-metal oxide interface. The mask aperture is then used to form a narrow gate electrode, and the gate electrode itself is used as a mask to form a closely spaced (short channel) source and drain.
Regardless of the success of these approaches, it is apparent that this exemplary prior art is not directed to the problem of decreasing channel width.